Editing Tar pit (section)

== Chemistry ==
Tar pits are pools of asphalt. However, at the beginning of their formation, they were not always sticky and dense. The pools were composed of [[crude oil]] that originated below Earth's surface. Crude oil is a mixture of [[heteroatom]] compounds, [[hydrocarbon]]s, [[metal]]s, and [[inorganic compound]]s.<ref name=":2">{{Cite web|title=Chemical Constitution of Crude Oil {{!}} FSC 432: Petroleum Refining|url=https://www.e-education.psu.edu/fsc432/node/5|access-date=2021-04-26|website=www.e-education.psu.edu}}</ref> Heteroatom compounds are organic molecules that contain elements that are not carbon or hydrogen, while hydrocarbons contain only carbon and hydrogen.<ref name=":2" /> Crude oil is less [[Viscosity|viscous]] than asphalt because it contains a higher percentage of light hydrocarbons.<ref name=":3">{{Cite web|title=Chemicals from Oil {{!}} S-cool, the revision website|url=https://www.s-cool.co.uk/gcse/chemistry/products-from-crude-oil/revise-it/chemicals-from-oil|access-date=2021-04-26|website=www.s-cool.co.uk}}</ref> Light hydrocarbons include the following [[alkane]]s: [[methane]], [[ethane]], [[propane]], and [[butane]].<ref name=":4">{{Cite web|title=2.5: Hydrocarbon Types {{!}} PNG 301: Introduction to Petroleum and Natural Gas Engineering|url=https://www.e-education.psu.edu/png301/node/833|access-date=2021-04-26|website=www.e-education.psu.edu}}</ref> These molecules have very low [[molecular weight]]s. Crude oils may also contain some inorganic impurities, such as CO<sub>2</sub>, H<sub>2</sub>S, N<sub>2</sub>, and O<sub>2</sub>.<ref name=":4" /> At the surface, these light molecules may evaporate out of the crude oil, leaving behind the heavier, stickier molecules. Asphalt, or bitumen, usually contains hydrocarbon molecule chains with 50+ carbon atoms.<ref name=":3" /> The longer the hydrocarbon chain, the more viscous it becomes, and the boiling point increases.<ref name=":3" />  

Evaporation is an important process in the formation of tar pits. A reservoir of light crude oil on Earth's surface can be reduced by up to 75% of the initial volume just after a few days, forming asphalt as the resulting product. For medium crude oils, the volume may be reduced by 40%.<ref>{{Cite journal|last=Fingas|first=Merv|date=1995|title=The Evaporation of Oil Spills|url=https://www.bsee.gov/sites/bsee.gov/files/osrr-oil-spill-response-research/120bw.pdf|journal=Proceedings of the Eighteenth Arctic Marine Oilspill Program Technical Seminar|via=McGill University}}</ref> Crude oils will evaporate differently depending on their chemical composition.<ref name=":2" /> The average composition of a bitumen sample by weight is 80.2% carbon, 7.5% hydrogen, 7.6% oxygen, 1.7% nitrogen, and 3.0% sulfur.<ref name=":1" />